This invention relates to an improvement of an insulated gate field effect transistor, and more particularly to a semiconductor device suitable to conversion into a high density integrated circuit (IC).
A generally known insulated gate field effect semiconductor device, for example, n channel MOS transistor (n channel Metal Oxide Semiconductor Transistor) has the following construction. This n channel MOST is constructed such that an n type source region and an n type drain region are formed in a p type silicon substrate in a manner spaced at a prescribed interval from each other; and a gate electrode is provided on a surface portion of the p type silicon substrate between the source and drain regions through a silicon dioxide film (SiO.sub.2) as an insulator. This n channel MOST is a voltage-controlled semiconductor device arranged to control a channel conductance produced between the source and drain regions by the voltage applied to the gate electrode, thereby to control the current flowing through the channel region between the source and drain regions.
In recent years, with regards to a semiconductor device using this type of element, it is demanded to miniaturize the MOST or element itself and especially to shorten the channel length thereof as a result of the demand for conversion of the device into a higher density integrated circuit. At present, however, it is very difficult, from the standpoint of MOST manufacture, to actually determine the channel length to a specified value with high accuracy. This means that upon MOST manufacture variations with occur in the channel length. In the case of the channel length of a presently used MOST, its variation do not go so far as to influence the element characteristics to so large an extent. But, the smaller the channel length, the higher the degree of its variation, and this poses a problem of having serious effect upon the MOST characteristics.
The effect, of the channel length variation upon the MOST element characteristics, namely the element characteristic variation resulting from the channel length variation, includes the variation in mutual conductance (gm) and the variation in gate threshold voltage (V.sub.T). The variation in the former or mutual conductance (gm) comes out as a change in the amount of current flowing in the transistor itself and, when viewed from the standpoint of the integrated circuit, comes out only as a change in the switching rate. The variation in the latter or gate threshold voltage (V.sub.T) becomes a cause of the logical erroneous operation of, for example, the digital circuit, resulting in raising a very serious problem.